As a transmission code used for an optical transmission system, the DMPSK (Differential Multiple Phase-Shift Keying) system with high nonlinear tolerance has been examined widely. In particular, a modulation system that combines a pulse carver with the DMPSK system and makes the inter-symbol optical intensity 0 is effective. Hereinbelow, in order to simplify the explanation, a description shall be given limited to the DQPSK (Differential Quadrature Phase Shift Keying) modulation system, which is multiple value phase modulation involving four values. In the following explanation, negation (NOT) of the first data signal Data 1 and the second data signal Data 2 is expressed by placing a bar over each term in the drawings, but in the present description, they are expressed as bar (Data 1) and bar (Data 2).
FIG. 10 is a block diagram that shows a typical configuration example of a DQPSK modulator 100 for producing a DQPSK signal, according to the conventional art. CW light that is input to the DQPSK modulator is split into two by a first coupler 1. The two CW lights that have been split are respectively input to a first optical phase modulating portion 2-1 and a second optical phase modulating portion 2-2. These first and second optical phase modulating portions 2-1 and 2-2 are normally constituted by MZI (Mach-Zehnder Interferometer)-type optical intensity modulators. The first and second optical phase modulating portions 2-1 and 2-2 have a function that relatively changes by π the optical phase in accordance with the logic of the first data signal Data 1, bar (Data 1) and the second data signal Data 2, bar (Data 2). The action of the first and second optical phase modulating portions 2-1 and 2-2 shall be described below.
The optical phase shifter 3 has an orthogonal bias electrode on which a third bias voltage described below is impressed. After the θ3 phase differential is added by this optical phase shifter 3, the outputs of the first and second optical phase modulating portions 2-1 and 2-2 are multiplexed by the second coupler 4, and output as a DQPSK signal. The best waveform is obtained by the θ3 being ±π/2 in the case of DQPSK. This corresponds to ¼ of the carrier wavelength. Since the wavelength is generally of the micrometer order, adjustment is extremely severe. Since the optical quality of the DQPSK signal is sensitive to errors of the optical phase shifter 3, it is extremely important to adjust the delay of the optical phase shifter 3 to the correct value.
Generally, this adjustment is performed by adjusting the third bias voltage (also called the orthogonal bias voltage) Vbias3 that is supplied from the third bias power supply 5 to the optical phase shifter 3. The optical phase shifter 3 is, in FIG. 10, arranged at a subsequent stage of the second optical phase modulating portion 2-2, but is not limited thereto. The optical phase shifter 3 may also be arranged at a subsequent stage of the first optical phase modulating portion 2-1, or may be provided at the subsequent stage of both the first optical phase modulating portion 2-1 and the second optical phase modulating portion 2-2. Hereinbelow, in order to simplify the description, the description shall be given for the case of the optical phase shifter 3 being arranged only at a subsequent stage of the second optical phase modulating portion 2-2.
Next, the operation of the first and second optical phase modulating portions 2-1 and 2-2 shall be described. As mentioned above, an MZI-type optical intensity modulator is generally used for the first and second optical phase modulating portions 2-1 and 2-2. The first and second optical phase modulating portions 2-1 and 2-2 are respectively driven by first data signal Data 1, bar (Data 1) and second data signal Data 2, bar (Data 2). These data signals Data 1, bar (Data 1) and Data 2, bar (Data 2) are NRZ (Non Return-to-Zero) signals of 2 values. The first and second drive amplifiers 6-1 and 6-2 amplify the first and second data signals Data 1, bar (Data 1) and Data 2, bar (Data 2) in the two types of the signals and inverted signals.
The amplified data signals Data 1, bar (Data 1) and Data 2, bar (Data 2) are impressed via the first drive signal electrode 7-1 and the second drive signal electrode 7-2 to each of the two arms that the first and second optical phase modulating portions 2-1 and 2-2 have, to produce a phase shift of ±φ1 and ±φ2. When the data signal changes from the L level to the H level, it is necessary to change the phase delay (φ1 and φ2 approximately by π. That is to say, the amplitude of the output and the inverted output of the first and second drive amplifiers 6-1 and 6-2 each must approximately be the half-wave voltage Vπ of the first and second optical phase modulating portions 2-1 and 2-2. Also, a DC voltage of ±Vbias1 and ±Vbias2 (data bias voltage, or differential bias) is produced by the first and second bias power supplies 8 and 9, and an optical phase shift of ±θ1 and ±θ2 is further added via the first data bias electrode 10-1 and the second data bias electrode 10-2.
The aforementioned half-wave voltage Vπ shall be described. The MZI-type optical modulator is an interferometer that splits one optical signal into two, and after applying a slight delay to one, multiplexes these two lights again. When the amount of this delay is changed, the interference light of the two lights during multiplexing either becomes constructive or deconstructive. ‘n’ is an integer, and the wavelength iso λ. When the delay is equivalent to nλ, the interference light becomes strongest, and when the delay is equivalent to (n+½), the optical interference is quenched. Since ‘n’ is an arbitrary integer, when the delay amount further increases, the increase or decrease of the interference light is periodically repeated.
In the MZI-type optical modulator, the delay of the MZI is controlled by an electrical signal from outside. Generally, there is a proportional relation between the voltage of the electrical signal and the increase or decrease of the delay. The amount of increase of the voltage required for the delay to change from nλ to (n+½) is called the half-wave voltage Vπ.
In the case of performing phase modulation with an MZI-type optical phase modulator, the total delay difference is modulated from (n−½)λ to (n+½)λ. For this reason, the electrical signal that is added to the modulator is required to have a voltage amplitude of 2×Vπ in total. In the case of using a differential input such as shown in FIG. 10, a voltage amplitude of Vπ may be imparted to each of the Data 1, bar (Data 1) and Data 2, bar (Data 2).
FIG. 11 is a diagram for describing the operation of the first optical phase modulating portion 2-1 by the prior art. In the following explanation, a non-essential optical delay that occurs from the optical phase change at the time of passage through the coupler, and the optical waveguide other than the interferometer is ignored. The electric field of light that has passed the first arm of the first optical phase modulating portion 2-1 is defined as E1a, the electric field of light that has passed the second arm as E1b, and the electric field of light that has been output from the first optical phase modulating portion 2-1 and just before being multiplexed by the second optical coupler 4 as E1 (refer to FIG. 10). The optical electric fields of light E1a, E1b, and E1 are sine waves that oscillate with the carrier frequency ωc. These optical electric fields of light E1a, E1b, and E1 are denoted by a dashed line, a dashed dotted line, and a solid line in FIG. 11, respectively, with time shown along the horizontal axis.
Case (1)
The case of φ1+θ1=0 is shown in the topmost row of FIG. 11. At this time, the amplitude of the electric field E1 of the output of the first optical phase modulating portion 2-1 is a maximum. The phases of the optical electric fields are in agreement at this time, with vertical arrows shown at the portions having the same phase for descriptive purposes.
That is to say, Case (1) shown in FIG. 11 shows the following case.
First arm: φ1+θ1=0
Second arm: −φ1−θ1=0
optical phase (relative value)=0
optical intensity of E1 (relative value)=|E1|2=1
Case (2)
The case of φ1+θ1=π/4 is shown in the second row of FIG. 11. The optical electric fields E1a, E1b, are symmetrically shifted to the left and right on the time axis, and so the arrows are divided from left to right. The amplitude of the multiplexed optical electric field E1 falls. However, since the shift amount is left-right symmetric, the position of the peak of the optical electric field E1 is unchanged from the case of φ1+θ1=0.
That is to say, Case (2) that is shown in FIG. 11 shows the following case.
First arm: φ1+θ1=π/4
Second arm: −φ1−θ1=−π/4
optical phase (relative value)=0
optical intensity of E1 (relative value)=|E1|2=0.5
Case (3)
The case of φ1+θ1=π/2 is shown in the third row of FIG. 11. Since optical electric fields E1a and E1b are opposite phases, the optical electric field E1 is quenched.
That is to say, Case (3) that is shown in FIG. 11 shows the following case.
First arm: φ1+θ1=π/2
Second arm: −φ1−θ1=−π/2
optical intensity of E1 (relative value)=|E1|2=0
Case (4)
The case of φ1+θ1=3π/4 is shown in the fourth row of FIG. 11. The optical electric field E1 is not quenched, but compared with the first row and the second row, the phase thereof is shifted by π.
Case (5)
The case of φ1+θ1=π is shown in the fifth row of FIG. 11. The amplitude of the optical electric field E1 again becomes a maximum. In the fifth row, compared to the first row, the optical phase is shifted by π.
That is to say, Case (5) that is shown in FIG. 11 shows the following case.
First arm: φ1+θ1=π
Second arm: −φ1−θ1=−π
optical phase (relative value)=π
optical intensity of E1 (relative value)=|E1|2=1
The power of the output of the optical phase modulating portion is proportional to the square of the absolute value of the electric field E1, with that being proportion to cos(φ1+θ1).
Normally, the two logical values of the first data signal Data 1, bar (Data 1) respectively are made to correspond to the state of the first and fifth rows of FIG. 11, respectively, and the DQPSK optical modulator is used in the state of becoming a maximum optical output intensity. However, the relationship between the output voltage of the first drive amplifier 6-1 and φ1 is not self evident, and may change over time. The first data bias electrode 10-1 and the phase difference ±θ1 that arises therefrom are used in order to correct this. For example, the output amplitude of the first drive amplifier 6-1 and the data bias voltage Vbias1 from the first bias power supply 8 are adjusted so as to become φ1+θ1=0 when the data signal is “0” and become φ1+θ1=π when the data signal is “1”. The same operation is performed for the second optical phase modulating portion 2-2.
As is clear in the foregoing explanation, provided the condition of supplying an antisymmetrical delay to the two arms is kept, the output lights of the first and second optical phase modulating portions 2-1 and 2-2 have the characteristics of (1) the optical intensity continuously changing and (2) the optical phase taking only two values that differ by π.
For reference, the case of driving the two arms asymmetrically shall be described. The “reference diagram” shown in the sixth row of FIG. 11 shows the case of asymmetrically changing the delay of the arms that is given in Case (5) of FIG. 11. The delay of the first and second arms was +π and −π in Case (5) of FIG. 11, but in the “reference diagram”, is changed to 0 and −2π. It is evident that the optical electric field E1 of the output light ends up becoming essentially the same as Case (1) of FIG. 11, and so phase modulation cannot be correctly performed.
That is to say, the reference diagram shown in FIG. 11 shows the following case.
First arm: φ1+θ1=0
Second arm: −φ1−θ1=−2π
optical phase (relative value)=0
optical intensity of E1 (relative value)=|E1|2=1
The optical electric fields E1 and E3 of the two lights that are multiplexed by the second coupler 4 (refer to FIG. 10) are expressed in writing in the manner of the following Equations (1) and (2), if φ1+θ1≡Φ1, and φ2+θ2≡Φ2. Hereinbelow, proportionality coefficients that are not essential are omitted.[Equation 1]E1=2×cos(Φ1)exp(iωc·t)  (1)[Equation 2]E3=2×cos(Φ2)exp(iωc·t+iθ3)  (2)
The average value of the output power of the DQPSK optical modulator is given by the following Equation (3).[Equation 3]|E1+E3|2=2+cos(2Φ1)+cos(2Φ2)+4·cos(Φ1)·cos(Φ2)·cos(θ3)  (3)
Next, the conditions that are required for control of the data bias voltages Vbias1 and Vbias2 shall be described. As stated above, with regard to the adjustment of the data bias voltages Vbias1 and Vbias2, the object is for the values of Φ1 and Φ2 to take the two values of 0 or π in accordance with the sign of the data. The adjustment of the data bias voltages Vbias1 and Vbias2 may be controlled so as to maximize the second term and third term of Equation (3). As is well known, this can be achieved by performing low-frequency dithering on the data bias voltages Vbias1 and Vbias2, synchronously detecting the dither frequency component of the output optical power of the DQPSK modulator, and feeding back the result to the data bias voltages Vbias1 and Vbias2.
Letting the dithering frequency be ωd, and the dither amplitude be Ad, the aforementioned Φ1 and Φ2 are replaced with Φ1+Ad×sin(ωd1·t) and Φ2+Ad×sin(ωd2·t).
Equation (3) is rewritten as shown in the following Equation (4).
                                              ⁢                  [                      Equation            ⁢                                                  ⁢            4                    ]                                                                                                                                    E                1                            +                              E                3                                                          2                =                  2          +                      cos            ⁡                          (                                                2                  ⁢                                      Φ                    1                                                  +                                  2                  ⁢                                      A                    d                                    ×                                      sin                    ⁡                                          (                                                                        ω                                                      d                            ⁢                                                                                                                  ⁢                            1                                                                          ·                        t                                            )                                                                                  )                                +                      cos            ⁡                          (                                                2                  ⁢                                      Φ                    2                                                  +                                  2                  ⁢                                      A                    d                                    ×                                      sin                    ⁡                                          (                                                                        ω                                                      d                            ⁢                                                                                                                  ⁢                            2                                                                          ·                        t                                            )                                                                                  )                                +                      4            ·                          cos              ⁡                              (                                                      Φ                    1                                    +                                                            A                      d                                        ×                                          sin                      ⁡                                              (                                                                              ω                                                          d                              ⁢                                                                                                                          ⁢                              1                                                                                ·                          t                                                )                                                                                            )                                      ·                          cos              ⁡                              (                                                      Φ                    2                                    +                                                            A                      d                                        ×                                          sin                      ⁡                                              (                                                                              ω                                                          d                              ⁢                                                                                                                          ⁢                              2                                                                                ·                          t                                                )                                                                                            )                                      ·                          cos              ⁡                              (                                  θ                  3                                )                                                                        (        4        )            
If the Vbias1 and Vbias2 are adjusted so that the ωd1 component and ωd2 component that are included in the modulator output light become 0, Φ1 and Φ2 become 0 or π, and it is possible to attain the object.
In the adjustment of the orthogonal bias voltage Vbias3, the object is to make the value of θ3 to be ±π/2. In order to achieve this object, a constitution has been proposed that performs low-frequency dithering on the orthogonal bias voltage Vbias3, synchronously detects the dither low-frequency component of the DQPSK modulator output light power, and feeds back the result to the orthogonal bias voltage Vbias3. There exists technology that performs bias control by superimposing a dither signal on the bias voltage (for example, refer to Patent Document 1).
As a constitution that similarly performs adjustment of the orthogonal bias voltage Vbias3, there exists technology that adds low-frequency dithering of different frequencies to both of the data bias voltages Vbias1 and Vbias2 to detect the sum frequency component or the differential frequency component from the DQPSK output light power, and by using this performs adjustment of the orthogonal bias voltage Vbias3 
As a different approach for performing adjustment of Vbias3, there also exists technology that adds low-frequency dithering of the same frequency but different phases to both of the data bias voltages Vbias1 and Vbias2, detects the double-wave component of the dither frequency from the DQPSK output light power, and by using this performs adjustment of the orthogonal bias voltage Vbias3.
A constitution has also been proposed that, using a high-speed photo detector, detects the change of each bit of the fourth term of Equation (3), and performs detection of the fluctuation of θ3 and performs adjustment of the orthogonal bias voltage Vbias3. Specifically, control by the peak detection circuit of the DQPSK modulator output light, or Costas loop are equivalent to this.